Table of contents

From a single crystal of nickel two specimens have been prepared in the form of prolate spheroids whose major axes lie along the [100] and [111] crystallographic directions. By the use of a special capacitance bridge arrangement described elsewhere, the longitudinal magnetostriction of each specimen has been measured over the temperature range -180°C to 360°C.

The results obtained indicate that at low temperatures the magnetostriction below saturation varies with magnetization approximately in the manner given by the domain treatment suggested by Heisenberg, and that with increasing temperature there is a steady departure from the calculated values. The saturation magnetostriction was found to depend on temperature in a way which did not correspond to any known theory.

The combinatorial method of Kac and Ward is used to derive the partition function of an infinite two-dimensional triangular Ising Lattice. The result agrees with that obtained by algebraic methods.

The method of atoms in molecules is investigated in detail for the case of covalent-ionic resonance in the ground state of the hydrogen molecule. It is shown that the method is reliable if, and only if, different orbitals are used to approximate atomic and ionic states of the same molecule.

The Mott cross section for the Coulomb scattering of high-energy electrons and positrons by atomic nuclei has been expressed by McKinley and Feshbach in terms of series in powers of Z/137, up to terms in Z⁴, where Z is the atomic number of the scattering nucleus. The coefficients of these series, which were evaluated numerically, have been re-calculated and some minor errors corrected. Further coefficients have been calculated to enable application of the method to heavy elements and the resulting cross section, expressed as a ratio to the Rutherford cross section, is given in the form of a single power series.

Calculations have been made of the bremsstrahlung spectrum emitted in different directions following electron-nuclear collisions, allowing for the finite distribution of the nuclear charge.

Calculations were carried out for charge distributions of the uniform sphere, spherical shell, gaussian and exponential forms, as well as for a uniform spherical core with a distribution falling off exponentially at the surface. In each case the parameters of the distribution were chosen to agree with the elastic electron scattering data at 16 MeV.

The modification of the angular distribution obtained from the Bethe-Heitler formula for a point nucleus is large for electron energies above 20 MeV.

The Landau-Zener formula for the transition probability arising from the pseudo-crossing of potential energy curves is applied to the processes H^-(1s)² + H⁺ → H(1s) + H(2s or p), H^-(1s)² + H⁺ → H(1s) + H(3s, p or d) and H(1s) + H(3s, p or d) → H(1s) + H(2s or p) allowance being made for the effect of the orbital degeneracy. Cross sections and rate coefficients are given over a wide range of impact energies and temperatures. The calculations show that the electron transferred in slow encounters between H^- and H⁺ ions falls mainly into the third quantum level. Recombination thus leads to the emission of the first Balmer line. The de-activation process studied is of interest in that it provides an illustration of a type of mechanism whereby electronic energy can readily be converted into translational energy in thermal collisions.

A stable β-detecting proportional counter has been constructed and used in the β-γ coincidence method to calibrate a Curie chamber substandard for ²⁴Na γ-ray sources. The mg Ra equivalent of ²⁴Na is found to be 0.333 ± 1.5% mc. This calibration leads to a revised value of the deuterium photodisintegration cross section at 2.76 MeV (15.05 ± 0.5 barns).

The methods of a previous paper are applied to the analysis of the reaction data on the lithium isotopes in terms of an intermediate coupling shell model of the nucleus. It is found that, to within the expected uncertainties, all of twelve data are consistent with the predictions of the model. The mean value of the intermediate coupling parameter a/K is about 3. As an incidental result, it is found that the β-decay data cannot be fitted with the old interaction mixture based on mirror transitions, but requires a new mixture based on the ¹⁴O Fermi transition.

The results reported in a previous paper on the nuclei ¹³C and ¹³N are extended in three ways: (i) The β-decay ft-value for the allowed mirror transition has been examined in intermediate coupling and is shown to be in excellent agreement with the previous results. (ii) Some properties of the nuclei have been recalculated with different assumptions about the central force between nucleons. It is found that the previous results are not very much changed, even when the central force mixture is quite severely modified. (iii) Some discussion is given about the even parity states of ¹³C and ¹³N. In particular the application of the ` unique parent ' concept to these states, and its justification, is considered.

Coincidences are used in studying the ¹¹B(p, α)⁸Be (α)⁴He reaction to infer directional correlations between successively emitted α-particles. From this the spin of the ground state of ⁸Be is found to be zero and its break-up energy 90 ± 5 keV. The absence of any measurable loss of kinetic energy in the ⁸Be nucleus when recoiling into a dense material enables us to place an upper limit on its half-life of 4 × 10^-15 sec, which confirms the assignment of zero spin.

Co-operative appearance of the rotation of hydrogen molecules in the solid phase is described using a semi-classical theory. The order of magnitude of the restriction to the rotation which is experienced by a molecule at the lowest temperature is estimated to be about 4°K using several different types of experiments, and the results are in reasonable agreement with each other. For the case of pure ortho-hydrogen a calculation of the potential energy and the anisotropy based on intermolecular forces is carried out in the Appendix.

An investigation is made of Weizsacker's correction to the Thomas-Fermi statistical treatment of the many-body problem. Numerical solutions of Weizsäcker's equation were obtained for the isotropic harmonic oscillator and the step potential in plane symmetry. These particular potentials were chosen as approximations to nuclear potentials. In the case of the harmonic oscillator, the error in the energy is an order of magnitude greater than for the Thomas-Fermi equation, but a reduction in the magnitude of the Weizsäcker correction term by a factor of fraction one-eighth gives substantial improvement over the Thomas-Fermi solution. The step potential also shows that the Weizsäcker term is too great, but the reduction factor necessary to give substantial improvement is between ½ and 1. It is concluded that the Weizsäcker correction term is not reliable as such, but that a reduction in the magnitude of the term may give plausible solutions for the density. The reduction factor depends, however, upon the form of the potential.

Born's approximation is applied to calculate the cross section for detachment of an electron from a negative ion of hydrogen in collision with a helium atom as a function of the energy of the ion. The results are compared with observed data and show agreement to within a factor of two or so over an energy range from 100 to 20 000 eV.

The results of an earlier publication are generalized to take account of the effect of the Coulomb field on the differential cross section in a deuteron stripping reaction. The results, which are expressed in analytical form, have been compared in detail with the observations in two reactions using ⁹Be as target nucleus at low energies.